Photograph: Jonathan Gregson
Preheat the oven to 150°C, fan 130°C, gas 2. For the base, place the biscuits in a large bowl and crush with your hands or a rolling pin. Add the butter and mix to a sandy paste. Press firmly and evenly into a 20cm square baking tin lined with baking parchment. Leave in the fridge to set.
Meanwhile, put the dark chocolate and butter in a heatproof bowl over a pan of simmering water and melt, stirring occasionally with a wooden spoon. Remove from the pan when melted.
Use an electric mixer to whisk 2 whole eggs plus an extra egg yolk and sugar until thick and pale (save the egg white for another recipe). This will take 15-20 minutes. Gently fold the chocolate mixture into the eggs, then the chopped chestnuts, figs and white chocolate. Spread evenly over the biscuit base and bake on the middle shelf of the oven for 25-30 minutes. When ready, a skewer inserted in the bar will come out with lots of gooey crumb attached, and there will be a light crust on top. Cool, then refrigerate until set.
Remove from the tin and peel off the baking parchment.
Cut into bars (or tiny squares to serve with good coffee) and dust lightly with cocoa powder. Let them come to room temperature before serving.
Get ahead: make the bars up to 3 days ahead; they can also be frozen.